To add a special character using a specific font, you have to do the followings. For example, if you want to add a check symbol, you have to add the following in your docbook file.

<!-- Insert the check symbol -->
<symbol role="ZapfDingbats">&check;</symbol><!-- in named entity -->
<symbol role="ZapfDingbats">&#x2713;</symbol><!-- in hexadecimal  -->
<symbol role="ZapfDingbats">&#8658;</symbol><!-- in decimal -->

In your customization layer, you tell docbook to use ZapfDingbats font-family for symbol element with ZapfDingbats role as follows:

<xsl:template match="symbol[@role = 'ZapfDingbats']">
  <fo:inline font-family="ZapfDingbats">
    <xsl:apply-templates/>
  </fo:inline>
</xsl:template>

With this process, you can add as many fonts as you want by simply using different role attributes.

References

• http://fet.aics.ru/doc/docbook/xsl/SpecialChars.html
• DocBook Character Entity Reference